Gas chromatography is based on the premise that the combination of analytes making up a sample introduced into a column within a gas chromatograph separate as they flow through the column at different rates and subsequently exit the column after different retention times.
In GC/MS analyses today, it is typical to analyze an unknown sample in a scanning mode, and then attempt to identify the resulting chromatographic peaks by mass spectral library searching. Such searching attempts to match a spectrum of the unknown analyte or combination of analytes to one or more spectra contained in a large mass spectral library. It is normal for software associated with the MS detector to report multiple matches (typically 10-20) with a number from 1 to 99 representing the closeness of the match. Retention time has not been employed as a qualifier for matching the analytes, because variations between the column and operating parameters of the GC system used to form the mass spectral library and those of another GC system employed for identifying analytes of interest resulted in large variations in retention time. In particular, variations may be due to instrument calibration, atmospheric temperature and pressure changes, oven design, column length, diameter, film thickness, phase type, and column degradation. These variations may be compounded over time, as it typically takes from weeks to years to conduct all of the analysis required to complete a detailed mass spectral library. Very often, many analytes are reported with similar match quality numbers because mass spectral library searching alone often cannot identify an analyte unequivocally. A prior art method for locking retention time by calculating an adjustment to the column head pressure required to lock a GC system to another GC system results in the retention time of an analyte of interest on both GC systems being the same is illustrated in FIG. 1. Retention time locking is more fully disclosed in commonly assigned U.S. patent applications Ser. No. 09/036658, filed on Mar. 6, 1998 entitled "Automated Retention Time Locking", Ser. No. 08/728,868, filed on Oct. 10, 1996 entitled "Automated Retention Time Locking", and commonly assigned U.S. patent application Ser. No. 08/846,977, filed on Apr. 30, 1997 entitled "Method for Sample Identification Using a Locked Retention Time Database" all of which are hereby incorporated by reference.
It is usually very tedious and impractical to search an entire chromatogram for analytes of interest in a large list. It would be advantageous if the analyst already knew where each analyte of interest will elute in the chromatogram if present, and extract ions inside a narrow window at that retention time to confirm the presence of the analyte. In this manner, retention time is employed as an orthogonal piece of information used to identify an analyte.
It would be advantageous to develop a retention time locked spectral database based on the results obtained using Mass Spectrometry (MS), Infrared detection (IR), Atomic Emission detection (AED) or other spectral detectors which are indexed by retention time of target analytes, their significant spectral data and ratios. For example, a mass spectral database could provide for identification of target analytes through the matching of significant mass spectral ions and the mass ion ratios with retention times in the retention time-locked spectral database if another GC system having the same column and operational parameters could be locked to the reference GC system. There is a need for a method that can use both retention time and spectral information to identify target analytes of interest.